A fundamental problem in the fabrication of GaN-based light-emitting diode (LED) chips is that the maximum attainable electrical conductivity of p-doped layers, especially of p-doped GaN or AlGaN layers, is not sufficient, in conventional LED chips made of other material systems, to effect the spread of current over the entire lateral cross section of the chip, since the front contacts normally used in such chips are made to cover only a fraction of the front of the chip in order to bring about the highest possible decoupling of radiation.
Growing the p-type layer on an electrically conductive substrate, which would make it possible to impress a current over the entire lateral cross section of the p-type layer, does not furnish economically justifiable results. The reasons for this are that fabricating electrically conductive lattice-matched substrates (e.g. GaN substrates) for the growth of GaN-based layers involves high technical expenditure, and growing p-doped GaN-based layers on non-lattice-matched substrates suitable for undoped and n-doped GaN compounds does not yield adequate crystal quality for an LED.
A known approach for combating the above-cited problem is to cover the full area of the side of the p-type layer facing away from the substrate with a contact layer that is transparent to the radiation, or with an additional, readily conductive layer to effect current spread, provided with a bonding contact.
However, the first-cited proposal has the disadvantage that a substantial portion of the radiation is absorbed in the contact layer. The second-cited proposal requires an additional method step that increases production expenditure.
Japanese abstract JP 10-150 220A discloses a light-emitting semiconductor component in which an n-GaN semiconductor layer, a light-emitting layer and a p-GaN semiconductor layer are deposited sequentially on an n-GaN substrate. A p-electrode is arranged on the topside of the p-GaN semiconductor layer, covering it substantially completely.